Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, application specific integrated circuits (ASICs), and other specific functional circuits. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, and power metal oxide semiconductor field effect transistor (MOSFET).
Semiconductor devices perform a wide range of functions such as signal processing, high-speed calculations, transmitting and receiving electromagnetic signals, controlling operation of electronic devices, and controlling movement of mechanical devices. Semiconductor devices are found in the fields of communications, power conversion, mechanical control, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
A microelectromechanical system (MEMS) is often used with the above semiconductor devices. For example, the MEMS can be a comb actuator, lens for cell phone camera, moveable mirror, accelerometer, or gyro. The MEMS may exhibit a capacitance from two electrodes or elements with a dielectric medium between the electrodes. The capacitance of the MEMS changes with the relative displacement or distance between the two electrodes. In the case of a lens for a camera, capacitance changes by movement of the focal point of the lens, or by movement of the lens with respect to a sensor, i.e., capacitance varies with displacement of the lens. The movement or displacement of the lens can be determined by measuring the capacitance of the MEMS. The present state and operation of the MEMS can be controlled by measuring changes in the effective capacitance.
FIG. 1 shows a conventional block and schematic diagram of measurement circuit 10 for measuring the capacitance of a MEMS, represented by variable capacitor 12 coupled between node 14 and terminal 16 operating at a ground potential. Charge pump 20 is coupled through connecting circuit 22 to node 14 to change the voltage across capacitor 12, which creates an electric field that imposes a force on the MEMS, e.g., to induce movement of one electrode of the MEMS capacitor. The value of MEMS capacitor 12 is then measured to determine the amount of the displacement or the new position of the electrode of the MEMS capacitor caused by application of the charge pump voltage. Connecting circuit 22 can be a transistor, resistor, or electronic switch. Capacitor 24 is coupled between node 14 and node 26, and capacitor 28 is coupled between node 14 and node 30. Reset circuit 32 can be a transistor or electronic switch coupled between node 26 and node 30. Pulse or step generator 34 has an output coupled to a non-inverting input of amplifier 36. The inverting input of amplifier 36 is coupled to node 26, and the output of amplifier 36 is coupled to node 30 at output terminal 38 of measurement circuit 10 to provide an analog output signal as a representative measurement of the value of capacitor 12.
Measurement circuit 10 provides an analog measurement of MEMS capacitor 12, i.e., the value of capacitor 12 is to be determined. Assume connecting circuit 22 and reset circuit 32 are closed or low impedance. The voltage from charge pump 20 is applied to capacitor 12 to cause displacement or change of state of the MEMS. Capacitors 24 and 28 isolate amplifier 36 from the high voltage from charge pump 20 on node 14, i.e., the voltage required to displace or change state of the MEMS can be greater than the breakdown voltage of the active amplifier. Capacitor 28 is coupled to node 14 to inject charges into MEMS capacitor 12. Capacitor 24 senses the change in the voltage at node 14 (V14). Since reset circuit 32 is low impedance, the voltage of the inverting input of amplifier 36 is substantially equal to the output voltage of the amplifier, which is equal to the voltage at the non-inverting input of the amplifier from pulse generator 34.
Connecting circuit 22 and reset circuit 32 are opened or set to high impedance, i.e., connecting circuit 22 and reset circuit 32 are disabled during the measurement phase. Pulse generator 34 provides a pulse or step function VP to the non-inverting input of amplifier 36. Note that the voltage at the inverting input of amplifier 36 will be substantially the same as VP applied to the non-inverting input of the amplifier. The output of amplifier 36 changes to make the inverting input of the amplifier follow VP. The output voltage of amplifier 36 changes based on a ratio of the capacitors and VP, i.e., the output voltage of amplifier 36 in response to VP is impressed through capacitor 28 to cause a change in V14 as a function of capacitor 12, which is then measured through capacitor 24 across the inputs of the amplifier and provided at the output of the amplifier. The value of capacitor 12 (displacement or state of the MEMS) is unknown due to the force applied by charge pump 20. Yet, the value of capacitor 12 can be determined from the change in V14 as a function of capacitor 12 and provided as the analog output voltage of amplifier 36 at output terminal 38. The analog output voltage of amplifier 36 changes with the value of capacitor 12.
FIG. 2 shows another conventional block and schematic diagram of measurement circuit 40 for measuring the capacitance of a MEMS, represented by variable capacitor 42 coupled between node 44 and terminal 46 operating at a ground potential. Charge pump 50 is coupled through connecting circuit 52 to node 44 to change the voltage across capacitor 42, which creates an electric field that imposes a force on the MEMS, e.g., to induce movement of the lens. The value of MEMS capacitor 42 is then measured to determine the amount of the displacement or the new position or state of the MEMS caused by application of the charge pump voltage. Connecting circuit 52 can be a transistor, resistor, or electronic switch. Capacitor 54 is coupled between node 44 and node 56, and capacitor 58 is coupled between node 44 and node 60. Reset circuit 62 can be a transistor or electronic switch coupled between node 56 and node 60. Pulse or step generator 64 has an output coupled through capacitor 66 to an inverting input of amplifier 68. The non-inverting input of amplifier 68 receives DC reference voltage VREF1. The inverting input of amplifier 68 is coupled to node 56, and the output of amplifier 68 is coupled to node 60 at output terminal 70 of measurement circuit 40 to provide an analog output signal representative measurement of the value of capacitor 42.
Assume connecting circuit 52 and reset circuit 62 are closed or low impedance. The voltage from charge pump 50 is applied to capacitor 42 to cause displacement or change of state of the MEMS. Capacitors 54 and 58 isolate amplifier 68 from the high voltage from charge pump 50 on node 44, i.e., the voltage required to displace a MEMS element can be greater than the breakdown voltage of the active amplifier. Connecting circuit 52 and reset circuit 62 are opened or set to high impedance, i.e., the connecting circuit and reset circuit are disabled during the measurement phase. Capacitor 58 is coupled to node 44 to inject charges into MEMS capacitor 42. Capacitor 54 senses the change in the voltage at node 44 (V44). Pulse generator 64 provides a pulse or step function VP through capacitor 66 to the inverting input of amplifier 68. The output voltage of the amplifier 68 changes based on a ratio of the capacitors and VP. Thus, the value of capacitor 42 can be determined from the variation of the analog output voltage of amplifier 68 and made available at output terminal 70.
The measurement of the MEMS capacitor uses conventional analog circuits, e.g., amplifier 36 or 68. The analog measurement value of the MEMS capacitor from amplifier 36 or 68 must be converted to a digital format, e.g., by an analog-to-digital converter (ADC) within the semiconductor die containing the measurement circuit or externally for further processing by digital circuits. The analog measurement of the MEMS capacitor may lack resolution needed in some applications.